The present invention relates to a ferrofluid seal apparatus and, in particular, to a ferrofluid seal apparatus suitable for a bearing, motor and the like.
FIG. 9 is a cross-sectional view of a conventional seal apparatus. The seal apparatus of FIG. 9 comprises a shaft 1, a housing 2, and an annular magnet 3. The shaft 1 is inserted coaxially into the housing 2 which is cylindrical. The annular magnet 3 is fixed to the inner side of an opening in the housing 2. Magnetic fluid 4 is retained in a small radial gap between the inner side of the annular magnet 3 and the outer side of the shaft. The North-South Pole of the annular magnet 3 is polarized axially, so as to form a magnet circuit with the shaft 1. The magnet circuit causes the magnet fluid 4 to be retained in the radial gap.
Normally, the amount of magnetic fluid 4 provided is somewhat more than the capacity of the radial gap. The magnetic fluid 4 adheres to the sides of the annular magnet 3 by virtue of a surface tension operation.
In the above-described seal apparatus, the shaft 1 is rotated, while the housing 2 is stationary. In other words, the shaft 1 should not be held stationary and the housing 2 should not be rotated. In this regard, the following problem will occur if the above-described seal apparatus is used in the mechanism in which the shaft 1 is stationary and the housing 2 is rotated. When the housing 2 is rotated, the annular magnet 3 fixed to the housing 2 is also rotated. The amount of magnetic fluid 4 attracted by rotating the annular magnet 3 is much more than in the case of rotating the shaft 1. Greater centrifugal force is applied to the magnetic fluid 4. Here, a portion of the magnetic fluid 4 positioned in the radial gap tends to move toward the housing 2 along the sides of the magnet 3 as indicated with the arrows in FIG. 9. Thus, an improved structure for stably sustaining the magnetic fluid 4 within the radial gap has not been proposed in the bearing field.
Further, in the bearing field a structure for preventing the magnetic fluid from moving and splashing due to the rotation of the bearing has not been proposed. In the motor field, a seal apparatus showing a good sealing property has not been proposed.